This invention relates to air humidifiers employing an electrically operated steam generator and, more particularly, to the water tank and electrodes for such steam generators.
Air humidifiers including an electrical steam generator for producing a stream of steam which is dispersed into the air distribution system for a building are well known. The steam generator includes a water tank which is partially filled with water and a plurality of electrodes immersed in the water. Electrical voltage is induced on the electrodes, current passing through the water causes it to boil, and a stream of steam is discharged through an opening in the top of the tank. Current flow between the electrodes is directly porportional to the surface area of the electrodes covered by the water. A control system controls the operation of a valve which admits water into the tank to maintain the water at a level necessary to produce steam at a rate required to obtain the desired humidification. The control system also periodically opens a drain valve for the purpose of removing contaminants from the tank.
Tap water is normally used and the water hardness varies considerably from locale to locale. Minerals in the water, such as calcium carbonate, tend to deposit on the surface of the electrodes. These deposits act as an insulator and reduce the effectiveness of the electrodes. The hardness of the water affects its conductivity. Thus, the water level in the tank required to produce a predetermined amount of steam at a given voltage varies depending on the extent of mineral deposits on the electrodes and the conductivity of the water. During operation the water level must be gradually increased to compensate for the loss of effective electrode area from the mineral deposits.
With initial water levels in the tank substantially below about 1/3 the electrode height, minerals tend to rapidly deposit on the immersed portion of the electrodes and these deposits build up to the point where a blockage is created in the bottom of the tank, preventing the inflow and/or drainage of water. This can occur with considerable unused electrode surface above the water level which means that the effective life of the steam generator is shorter than possible. Also, the electrodes can become completely uncovered from time to time during the drain cycle. This can adversely affect the electrical control system which operates on the basis of the amount of electrode immersion.
With initial water levels over about 1/2 of the electrode height, the increasing water level required to compensate for mineral deposits during operation reaches a point where the volume above the water level is insufficient to produce the required steam within a shorter than optimum time period. Thus, it is desirable for the electrodes to be designed so they can produce the desired amount of steam at a given electrode voltage with about 1/3 of their height immersed in water at startup.
The current passing through the electrodes varies as the circumferential spacing or horizontal distance between the electrode varies. Accordingly, the electrode mounting arrangement desirably should be arranged to permit convenient adjustment of the horizontal distance between electrodes when required to compensate for the conductivity of the particular water to be used. Adjustable electrode mounting arrangements are known; however, they are not designed to insure that adjacent electrodes are maintained in parallel relationship and uniformly spaced when adjustments in the horizontal distance are made.
U.S. Pat. No. 3,761,679 discloses the use of tapered electrodes having a complex geometric shape such that, as the water level increases, the surface of the electrodes covered by water increases geometrically and the horizontal distance between the electrodes decreases. With such an arrangement, the effect of differences between the conductivity of water is offset by relatively small increases in the water level which produces a substantial increase in the current flow due to the combined effect of the increased electrode area and the decreased horizontal distance between the electrodes and a proportional increase in the amount of steam generated. However, there is no provision for adjusting the horizontal distance between the electrodes as may be required for certain water conductivities in order to permit start up with an optimum initial wafer fill as discussed above.